Separating one silicon layer from another silicon layer by fracturing a thin and mechanically weak/fragile intermediate silicon layer has been widely known in making silicon-on-insulator (SOI) wafers for producing semiconductor devices. However, prior methods have several drawbacks. For example, most prior art methods require a planar intermediate layer separating the device layer and the substrate layer. U.S. application Ser. No. 11/868,489 having common inventor, Mehrdad Moslehi, of the present disclosure discloses a 3-D thin-film semiconductor device where prior art manufacturing methods may not be suitable.
Instead of having a flat porous silicon layer, the honeycomb 3-D TFSS and the template wafer comprise three-dimensional microstructures with high-aspect-ratio deep trenches made into the silicon template. As a result, the effective interface area between porous silicon layer and nonporous silicon layers is at least five times larger than that of a flat substrate. The large interface area per unit volume increases the magnitude of external energy/force that is required for fracturing the porous silicon layer. Prior art methods may not be suited to fracture the porous silicon, while mitigating damage to both the template and 3-D TFSS.
In addition, most release methods in the prior arts require a mechanical supporting plate bonded or attached by adhesive on top of the thin epitaxial silicon layer to be released. In addition to serving as a mechanical support, the bonded top plate may also absorb the external energy and generate a stress on the layer to be released. Without the top supporting plate, many of the prior art release methods are either less effective or cause mechanical damage to the released thin-film. U.S. patent application Ser. No. 11/868,489, entitled “METHODS FOR MANUFACTURING THREE-DIMENSIONAL THIN-FILM SOLAR CELLS” by Mehrdad Moslehi and incorporated by reference herein, discloses a 3-D TFSS which is not conducive to the use of a top supporting plate for release and post-release processes because: (i) it is not convenient to bond a supporting plate on top of the square 3-D TFSS to be released while preventing the supporting plate from attaching to the wafer surface outside of the 3-D TFSS square; (ii) it is difficult to de-bond the supporting plate from the released 3-D TFSS. In the case that the bonding adhesive has to be wet removed, extensive cleaning may need to be performed to prevent adhesive contaminations to the honeycomb surfaces.
Further, most of the release methods in the prior arts initiate a single separation front in the porous silicon layer at the beginning of release that propagates through the entire wafer to complete the release. In most cases, the separation front starts from the wafer perimeter and the released portion of the epitaxial silicon layer curves upward as the separation progresses towards to the wafer center. Such a release mechanism works well for a planar release, however it does not work for the 3-D TFSS release for the following reasons: (i) because of its three dimensional structural design, the early released portion of honey-comb structure can not be tilted. A slight out-of-plane curving by an external force or an intrinsic stress will have the 3-D TFSS locked into the template and prevent a full release; (ii) larger external energy/force applied unevenly to the partially released and locked-in 3-D TFSS could cause mechanical damages. Therefore, the release energy/force should be uniform and applied in a well controlled manner for the 3-D TFSS release.
It is known that the mechanical strength of porous silicon depends on the porosity of the layer, and that porous silicon mechanical strength is sufficiently lower than that of non-porous silicon. As an example, a porous silicon layer having a porosity of 50% may have a mechanical strength about one-half of that of a corresponding bulk silicon layer. When a porous silicon layer is subjected to compressive, tensile, or shearing forces, it can be fractured, collapsed, or mechanically destroyed. A porous silicon layer, which has higher porosity, can be fractured with less applied stress.
One method for collapsing the mechanically weak porous silicon layer employs injecting the porous layer with a fluid. This method not only succumbs to the difficulties of the prior art method mentioned above, but is also complex and requires precise alignment of the fluid injection nozzle with the porous silicon layer so as not to damage the thin-film layer.
In another prior art method, a process of manufacturing a SOI wafer includes separating a wafer assembly into two wafers at a fragile silicon layer containing a high amount of hydrogen. The separation energy source can be selected from a group consisting of: ultrasound, infrared, hydrostatic pressure, hydrodynamic pressure, or mechanical energy. Also, yet another prior art method applies a force to a laminating material separating a nonporous silicon and porous silicon layer to separate the two layers.
Besides succumbing to the disadvantages mentioned previously, these methods may often damage the template layer which is undesirable for releasing TFSS substrate of U.S. application Ser. No. 11/868,489. Other advantages of the present disclosure may be apparent to those skilled in the art.
Patent applications U.S. Pat. Pub. No. 2008/0264477 and U.S. Pat. Pub. No. 2009/0107545 by common inventor Mehrdad M. Moslehi disclose methods of making solar cells using thin crystalline silicon substrates that require releasing/separation of the thin crystalline silicon substrates from reusable silicon templates. The purpose of the present disclosure is to provide methods and apparatus for releasing/separation of the said substrates from the corresponding templates.
According to the disclosed patent application, there exists a thin mechanically-weak layer that physically connects the thin substrate and template across their lateral interface. Examples of the thin mechanically-weak layer include, but are not limited to, a porous silicon layer having a uniform or multiple-level porosities. The separation of the substrate and template requires cleaving through the mechanically-weak layer between the substrate and the template by breaking off the micro-structures in the mechanically-weak layer. Due to the large lateral interface area between the thin substrate and template, it usually requires a large vertical pulling force to separate the substrate from the template by direct vertical pulling, in which case the micro-structures that connect the substrate and the template within the mechanically-weak layer are broken simultaneously. In many cases, the required direct-vertical pulling force may be larger than the chucking forces that hold the substrate and the template on the opposite sides during pulling. For example, vacuum chucks are used for chucking the substrate and the template from their opposite sides for pulling, and the required pulling pressure (force/unit contact surface area) to separate the substrate and the template may be larger than the vacuum chucking pressure of maximum 100 kPa. In addition, large pulling forces may create mechanical shock waves and cause the thin substrate to break during the pulling. Because semiconductor thin substrates are fragile in nature, they have tendency to crack with small mechanical shock or shearing impacts. Furthermore, bending and deflection with large angles during the releasing/separation process may also cause cracking of the thin substrate.
Semiconductor thin substrates are fragile in nature and they have tendency to crack with small mechanical shock or shearing impacts. In addition, the cracking of thin semiconductor substrates most likely initiates from their defect regions, especially edge defects such as micro-cracking at substrate edges. Therefore the prevention of cracking initiation at substrate edges is critical. In order to prevent cracking of a thin semiconductor substrate during the said substrate releasing/separation process, the understanding of the micro-structural characteristics of the buried mechanically-weak layer is important:
The mechanical strength of the micro-structures within the mechanically-weak layer is not uniform across attached substrate and template. There exist some regions in the said layer that are weaker than its surrounding regions. For example, in the case the mechanically-weak layer is a bi-level porous silicon layers that has a high porosity (60% to 80%) porous silicon layer on the template side and a low porosity (10% to 30%) porous silicon layer on the substrate side, the porosity values are not uniform from center to edge of the template. In addition, after baking at elevated temperatures and epitaxial silicon growth around 1000° C., the porous silicon layers are coalesce and/or disintegrated to various degrees across template surface. Therefore the mechanical strength of the said thin layer is not uniform from locations to locations within a wafer and across the wafer. FIGS. 42A and 42B illustrate the top and cross-sectional schematic views of the various mechanical strength regions in the mechanically-weak layer; the mechanical strength non-uniformity of the mechanically-weak layer may vary from wafer to wafer and from batch to batch; and during the releasing process, new weaker portions may be generated and existing weaker portions may be enlarged or further weakened by the direct releasing forces and indirect energy waves from the releasing process. In other words, the weakest mechanical strength portions at any given moment of the releasing process are generated and varied dynamically. FIGS. 43A and 43B illustrate the top and cross-sectional schematic views of the various mechanical strength regions corresponding to FIGS. 42A and 42B with newly and dynamically generated weaker portions in the mechanically-weak layer during the releasing process.
Current thin substrate releasing methods assume the mechanical strength of the mechanically-weak layer is uniform across the entire wafer (substrate and template assembly) and along the periphery of the substrate and template interfaces. With this assumption, the initial cleaving/releasing is randomly chosen from the wafer edge and the continuation of the cleaving is forced to propagate from this randomly chosen starting point. As a result, any substrate or substrate region at any given moment may not be released from the locations and the directions that have the weakest local mechanical strength in the mechanically-weak layer. Therefore the release yields are low and cracking of the thin substrate often happens.